add-watch

Adds a watch function to an agent/atom/var/ref reference. The watch
fn must be a fn of 4 args: a key, the reference, its old-state, its
new-state. Whenever the reference's state might have been changed,
any registered watches will have their functions called. The watch fn
will be called synchronously, on the agent's thread if an agent,
before any pending sends if agent or ref. Note that an atom's or
ref's state may have changed again prior to the fn call, so use
old/new-state rather than derefing the reference. Note also that watch
fns may be called from multiple threads simultaneously. Var watchers
are triggered only by root binding changes, not thread-local
set!s. Keys must be unique per reference, and can be used to remove
the watch with remove-watch, but are otherwise considered opaque by
the watch mechanism.